Mostly, one would imagine, like anyone else interested in the history and beliefs of a certain region, as an academic by looking at the earliest written evidence starting with that of the Ven Bede, through medieval literature etc or if not an academic, perhaps by reading the more readily available books compiled on the subject since the 19th century. It should be understood, however, that from an academic viewpoint, the early texts are all written by those from a different belief system. Bede was an English monk and most of the others were written in Latin by Christian monks so both were seeing or reporting what they saw through their own bias but then if you have studied religions for many years, I'm not trying to teach you to suck eggs as the saying goes.
The 'mystery school' you mention specifically, I've never heard of (sounds so enigmatic doesn't it? Mystery school!). One would have to ask the lady herself where she came by the information to set up her school but she does give some personal details here:
http://www.chalicecentre.net/bio.htm
A list of some related literature if you are interested:
http://en.wikipedia.org/wiki/Early_Irish_literature
http://www.mabinogion.info/
http://en.wikipedia.org/wiki/Bede
Major Celtic Manuscripts
Irish Manuscripts
The Cathach of Saint Columba (560-630 AD?; psalms & Irish rubrics)
The Book of Durrow (ca. 650?; [gospel]s)
The Stowe Missal (792-803; Mass-book)
The Book of Kells (8th C.?; gospels)
Domnach Airgid fragment (8th C.; gospel)
The Book of Armagh (10th C.; gospels)
The Book of the Dun Cow (11th/12th C.; [anthology])
The Book of Glendalough (1130; genealogies)
The Book of Leinster/Oakvale (ca. 1150; anthology)
The Black Book of Limerick (13th C; religious)
The Book of Fermoy/The Book of Roche (1373; anthology)
The Book of Ballymote (1391; anthology)
The Yellow Book of Lecan (1391-1401; anthology)
The Book of Hy Many (1394; anthology)
The Book of Lismore (1417; religious & secular anthology)
The Great Book of Lecan (1418; anthology)
The Leabhar Breac/Speckled Book (14th/15th C; religious)
The Red Book of Ormond (14th C; historical/administrative)
The Red Book of Ossory (13th?-15th? C; religious)
The Book of Flavus Fergusiorum (1437 - 1440)
The Book of Fenagh (1516)
The Red Book of the Earls of Kildare (16th C; historical)
Welsh Manuscripts
Harlian 3859 (9th C?; history) {Unnamed, but extremely important, as it contains the Historia Brittonum and Annales Cambriae.}
The Book of Llan Dav (1150; history/ecclesiatical)
The Black Book of Carmarthen (1250; poetry)
The Book of Aneurin (1250?; poetry)
The Hendregadredd Manuscript (1282-1330+; poetry)
The Black Book of Chirk (13th C; law)
The Book of Taliesin (1325?; poetry)
The Black Book of St. David's (1326; history/ecclesiastical)
The White Book of Rhydderch (1350; anthology)
The Red Book of Hergest (1382; anthology)
The White Book of Hergest (1450 C.; anthology; lost)
The Red Book of Talgarth (1400; poetry)
The Black Book of Basingwerk (15th C; poetry)
Scottish Manuscripts
The Book of the Deer (9th C. gospels)
The Red Book of Menteith (14th C; historical)
The Book of the Dean of Lismore (15th C. anthology)
The Black Book of Clanranald (17th C; poetry)
The Red Book of Clanranald (17th C; poetry)
The Black Book of Taymouth (1648; historical/genealogies)
